In a conventional mobile network, a data communication path between user equipments (full name: User Equipment, UE for short) needs to pass through a core network entity, that is, a serving gateway (Serving Gateway, SGW) or a packet data network gateway (full name: Packet Data Network Gateway, P-GW for short). When the UEs are close to each other and need to transmit data, the data still needs to be transmitted over a core network. This occupies a large amount of bandwidth and imposes a high requirement on a bandwidth amount, and therefore transmission efficiency is relatively low. Currently, a proximity service (full name: Proximity Services, ProSe for short) technology is usually used. This technology mainly implements a direct data exchange between UEs close to each other without using a network, provided that the UEs can discover each other. A specific transmission process may be: Remote (Remote) UE outside cell coverage communicates with relay (Relay) UE within the cell coverage area by using the ProSe technology, and the relay UE forwards, by using an existing LTE technology, data sent by the remote UE to an evolved NodeB (Evolved Node Base, ENB for short) and to a P-GW, so as to implement a relay function.
When an application server (Application Server) sends data to remote UE, because priorities of data sent by an application layer are different, application servers have different priorities at the application layer (Application Layer), and relay UE can parse only an Internet Protocol header (full name: Internet Protocol, IP Header for short), the relay UE cannot learn an application layer priority after receiving the data. As a result, when sending the data to the remote UE, the relay UE can only process the data according to a priority sequence of the data.
To resolve the foregoing problem, the following manner is generally used in the prior art: When obtaining data from an application server, a P-GW maps the data to different data bearers according to an application level priority. Each bearer has a corresponding service quality class identifier (full name: Quality Class Identifier, QCI for short). Relay UE obtains a corresponding priority according to a QCI of a bearer on which received data is located and a pre-configured mapping relationship between a QCI and a priority (Priority), and processes the data according to a priority sequence.
However, the priority is a fixed parameter of the application level, and therefore the foregoing mapping relationship is fixed. In a unicast (Unicast) data transmission process, a defined quantity of QCIs is insufficient to form a one-to-one correspondence with an application level priority. When receiving multimedia broadcast multicast service (full name: Multimedia Broadcast Multicast Service, MBMS for short) data, relay UE does not know a QCI of the MBMS data. Therefore, the existing transmission rule is inapplicable to downlink transmission of MBMS data and unicast data.